There are a number of well known contact devices used to measure the temperature of a continuously moving filament of wire. However, when the temperature of very small diameter wire is to be measured it is preferred to use noncontact techniques to avoid damage and/or breakage of the wire. Practically, the use of radiant pyrometric techniques is the most effective noncontact means of measuring the filament temperature.
A typical apparatus using pyrometric techniques is disclosed in U.S. Pat. No. 3,924,469. That patent is directed to measuring the thermal radiation emitted from a continuously moving wire using a pyrometer. The wire passes through a cylindrical metallic body having a reflective inner surface. A rotating mirror within the cylinder alternately directs the radiation from the heated wire and the reflected radiation from the cylinder walls to the pyrometer. The difference between the wire radiation and the wall radiation is an indication of the wire temperature.
Although such pyrometric techniques work well to determine temperatures of large diameter wires, when the wires have a very small diameter (e.g., 22 gauge), problems arise. The amount of thermal radiation emitted from a small area of the wire monitored by a pyrometer (e.g., less than 1/3 cm.sup.2) is relatively low. Additionally, the thermal radiation which is being emitted by the wire has not been utilized fully by prior art radiation pyrometers because the radiation reflected by the surroundings as well as a stray radiation influence the value obtained by such measurements in an unpredictable manner.
Accordingly, in order to improve the accuracy of a pryometric measurement, it is known to provide radiation absorbing material behind or around the wire in the vicinity of the pyrometer detecting apparatus. Although the use of such material improves the repeatability of the temperature measurement, some radiation will be reflected from the absorbing material in an unpredictable fashion resulting in measurements that are not accurately repeatable.
Thus, there is a need for an accurately repeatable, noncontact technique for measuring the temperature of a continuously moving filament.